1. Field of the Invention
This invention relates generally to a servomotor device and more specifically to a servomotor device for a brake booster for an automobile. A power piston in the servomotor device, which divides an interior portion of a housing into a constant pressure chamber and a variable pressure chamber with a diaphragm, comprises a hub having a control valve for controlling the pressure difference between the constant pressure chamber and the variable pressure chamber and a pressure plate of metal engaged with the hub and prevented from movement toward the constant pressure chamber with respect to the hub by engagement of an inner sleeve portion thereof with a shoulder located on the hub and facing toward the variable chamber.
2. Description of the Prior Art
A servomotor device is disclosed in U.S. Pat. No. 3,897,718. In such conventional servomotor device, a hub made of plastic resin is combined with a pressure plate made of metal for engaging an inner sleeve portion of pressure plate with a first groove formed on an outer peripheral portion of hub and a shoulder facing toward a variable chamber so as to prevent movement of the pressure plate toward the constant pressure chamber with respect to the hub and a second groove for retaining an inner bead of a diaphragm hermetically formed on inner side of inner sleeve portion so as to prevent movement of the pressure plate toward the variable pressure chamber by the diaphragm. However, the diaphragm can become damaged due to the force of the pressure plate, moving toward the variable pressure chamber with respect to the hub, which operates as a shearing force on the diaphragm. Furthermore, when the damaged diaphragm is removed, the diaphragm is oftentimes cut by a knife so as to separate the diaphragm from the power piston, whereby the connection of the pressure plate with the hub is released. Therefore it is very troublesome to exchange the diaphragm.